Hermetically closed package to be heated in a microwave oven

Abstract

A hermetically closed package enclosing a product comprises a wall formed of a packaging material including a first polymer film ( 7 ) and a second polymer film ( 10 ) being co-extensive with the packaging material, the first polymer film ( 7 ) is laminated to the second polymer film ( 10 ) by means of an adhesive layer ( 16 ). One of the polymer films is provided in a limited region thereof with a layer of a material generating heat when subjected to microwave radiation, i.e. a heat-generating layer ( 5 ) having susceptor effect. The heat-generating effect, i.e. the susceptor effect, of the heat-generating layer ( 5 ) is selected so as to melt, decompose, soften or weaken the first polymer film ( 7 ) and/or the second polymer film ( 10 ) in the limited region as seen transversely of the packaging material to such a degree that an opening is formed in the limited region.

1 . Hermetically closed package enclosing a product ( 2 ), especially a food product, to be heated in a microwave oven while being enclosed in said package, said package comprising a wall ( 3 ) formed of a packaging material including a first polymer film ( 7 ) and a second polymer film ( 10 ) being co-extensive with the packaging material, the first polymer film ( 7 ) having a first surface ( 8 ) and a second surface ( 9 ), the second polymer film ( 10 ) having a first surface ( 11 ) and a second surface ( 12 ), the first surface ( 11 ) of the second polymer film ( 10 ) being laminated to the second surface ( 9 ) of the first polymer film ( 7 ) by means of an adhesive layer ( 16 ), said package further comprising a layer of a material generating heat when subjected to microwave radiation, i.e. a heat-generating layer ( 5 ) having susceptor effect,
 characterised in that   a limited region of the first surface or the second surface is provided with the heat-generating layer ( 5 ), said heat-generating layer ( 5 ) being deposited or printed on first or second surface ( 8 ,  9 ) of the first polymer film ( 7 ) or to the first or second surface ( 11 ,  12 ) of the second polymer film ( 10 ) and having an extent substantially smaller than the extent of the wall ( 3 ) and having an electrical conductivity corresponding to a surface resistance of 4 to 9000 ohms per square measured according to DIN IEC 93, and that the packaging material is provided with a non-through-going or through-going perforation in the first polymer film ( 7 ) in the region of the heat-generating layer ( 5 ) as seen transversely of the packaging material, and wherein the susceptor effect of the heat-generating layer ( 5 ) is selected such that when subjected to a prescribed amount of microwave radiation, the first polymer film ( 7 ) and/or the second polymer film ( 10 ) are capable of melting, decomposing, softening or weakening to such a degree that the packaging material disintegrates to form a through-going opening there through in line with the perforation in the first polymer film ( 7 ).   
     
     
         2 . Package according  claim 1 , wherein the heat-generating layer ( 5 ) has an electrical conductivity corresponding to a surface resistance of 100 to 9000 ohms per square, alternatively 500 to 9000 ohms per square, alternatively 500 to 6000 ohms per square, alternatively 500 to 3000 ohms per square or alternatively 500-2000 ohms per square, measured according to DIN IEC 93. 
     
     
         3 . Package according to  claim 1 , wherein the first surface ( 8 ) of the first polymer film ( 7 ) is directed to an exterior of the package and the second surface ( 9 ) of the first polymer film ( 7 ) is directed to the interior of the package. 
     
     
         4 . Package according to  claim 1 , wherein the first surface ( 8 ) of the first polymer film ( 7 ) of the packaging material is an outer surface of the package. 
     
     
         5 . Package according to  claim 1 , wherein the second surface ( 12 ) of the second polymer film ( 10 ) of the packaging material is an inner surface of the package. 
     
     
         6 . Package according to  claim 1 , wherein the first polymer film ( 7 ) is an oriented film. 
     
     
         7 . Package according to  claim 1 , wherein the heat-generating layer ( 5 ) is arranged in an area outside of sealing seams ( 15 ) of the packaging material as seen transversely of the packaging material. 
     
     
         8 . Package according to  claim 1 , wherein the heat-generating layer ( 5   a ) overlaps at least in part a sealing seam ( 15 ) of the packaging material as seen transversely of the packaging material to provide an easy opening device. 
     
     
         9 . Package according to  claim 1 , wherein the perforation includes at least one slit having a length of at least 1 mm such as 3-25 mm, preferably 3 to 12 mm. 
     
     
         10 . Package according to  claim 9 , wherein the perforation includes two crossed slits. 
     
     
         11 . Package according to  claim 1 , wherein a non-through-going perforation extends through and has at least 10%, alternatively at least 20, 30, 40 or 50% of the thickness of a first polymer film. 
     
     
         12 . Package according to  claim 1 , wherein the heat-generating layer ( 5 ) is arranged and shaped so as to overlap any perforations in the packaging material as seen transversely thereof. 
     
     
         13 . Package according to  claim 1 , wherein the heat-generating layer ( 5 ) is arranged and shaped so as to not overlap any perforations in the packaging material as seen transversely thereof. 
     
     
         14 . Package according to  claim 1 , wherein the heat-generating layer ( 5 ) is a metallisation layer, especially provided by physical vapour deposition (PVS) and especially an aluminium layer. 
     
     
         15 . Package according to  claim 1 , wherein the heat-generating layer is a printing ink containing particles having susceptor effect such as graphite or carbon, indium oxide, tin oxide or a metal such as aluminium, silver, nickel, tin or stainless steel, preferably graphite or carbon particles or aluminium particles. 
     
     
         16 . Package according to  claim 1 , wherein the heat-generating layer ( 5 ) has an electrical conductivity corresponding to such a surface resistance that at least 24 W is absorbed by the heat-generating layer when the package is heated in a microwave oven. 
     
     
         17 . Use of a package according to  claim 1 , wherein the package and the product enclosed therein is subjected to heat treatment, and subsequent heating of the package with the product enclosed therein in a microwave oven. 
     
     
         18 . Use according to  claim 17 , wherein the heat treatment is pasteurisation. 
     
     
         19 . Use according to  claim 17 , wherein the heat treatment is sterilisation.